[wolf k]

blykins is correct, some one piece gaskets have raised sealing ribs on both sides, if you try to mate an embossed pan rail with a raised ribbed gasket on the pan side it is going to leak. Also, most one piece gaskets I have seen have metal rings around the bolt holes, this prevents deforming the gasket and pan by over-tightening. If your pan rail is already deformed from years of service then the metal rings will prevent the gasket from sealing and it will leak.